1. Field of the Invention
The present invention relates generally to devices for measuring metabolic parameters. More particularly, the present invention relates to apparatus and methods to generate rapid estimates of these parameters.
2. General Background and State of the Art
Metabolic chambers based on open circuit spirometry are used to determine the metabolic parameters of a patient within it. With the obesity epidemic of current concern, such information is of particular interest for infants and young children as a noninvasive tool to evaluate their metabolic status and food requirements. Metabolic chambers are also used to monitor the metabolism of non-humans such as farm animals (cattle, pigs, . . . ) and laboratory test animals (rats, mice, monkeys, . . . ). To be effective clinically, the chambers need to produce metabolic parameters quickly and reliably.
A metabolic chamber can be a rigid structure made of solid plastic or glass walls, or a flexible structure like the plastic film canopy placed over patients. To generate the metabolic parameters, the chamber air is steadily removed while fresh air enters, and the removed air is sampled to determine the fractions of CO2 and O2 and the air removal flow rate. Metabolic parameters are determined by combining these in equations describing the steady state condition of the chamber. These results are not valid until steady state is achieved, and this is dependent on the volume of the chamber and the entry and removal flow rates. In typical chambers designed for infants, steady state can take as long as an hour and, since during this time the metabolism of the patient can be changing, steady state may never actually exist.
The ability to determine metabolic parameters quickly while the chamber is not at steady state is preferred and would make it possible to reduce the time the patient is in the chamber and to produce more accurate metabolic parameter values. There is a need to provide an apparatus and method to accomplish this.
Commercially available metabolism monitoring devices may not enclose the patient, but use a mouthpiece through which the patient breathes into a mixing chamber or a small chamber enclosing only the head (Deltatrac II by Datex-Ohmeda). Commercial devices (Deltatrac II) are also used to monitor intensive care patients lying under large canopies having volumes large enough to be affected by steady state equilibration and that can be improved using the invention described herein.